1. Field of the Invention
The present invention relates to a plasma process apparatus and a plasma process method. More particularly, the invention relates to a plasma process apparatus typified by a plasma enhanced CVD system used in fabrication of semiconductor devices, photosensitive devices for electrophotography, line sensors for input of image, flat panel displays, imaging devices, photovoltaic devices, and so on, and to a plasma process method typified by a deposit film forming process, and etching and ashing processes by plasma CVD.
2. Related Background Art
In recent years, the plasma enhanced CVD systems are industrially employed practically in the fabrication processes of semiconductor devices or the like.
Particularly, the plasma enhanced CVD systems using the high frequency radio frequency wave of 13.56 MHz or the microwave of 2.45 GHz are widely used, because they can process materials for substrates, materials for deposited films, etc., regardless of whether conductor or insulator.
A parallel plate type apparatus using high frequency energy will be described as an example of the plasma enhanced CVD system, referring to FIG. 1. A cathode electrode (703) is placed through an insulating cathode electrode support (702) in a reaction vessel (701). A ground shield (704) is positioned around the cathode electrode (703) so as to prevent discharge from occurring between the side of cathode electrode (703) and the reaction vessel (701). A high frequency power supply or RF generator (711) is connected through matching circuit (709) and RF power supply line (710) to the cathode electrode (703). A film-deposited substrate of flat plate (706) subjected to plasma CVD is placed on an opposed electrode (705) disposed in parallel with the cathode electrode (703) and this film-deposited substrate (706) is kept at a desired temperature by a substrate temperature controller (not shown).
The plasma CVD is carried out as follows using the above apparatus. The reaction vessel (701) is evacuated to a high vacuum by a vacuum evacuator (707) and thereafter reaction gas is introduced into the reaction vessel (701) from a gas supply source (708) to maintain the inside of reaction vessel under predetermined pressure. Then RF power is supplied from the RF generator (711) to the cathode electrode (703) to generate a plasma between the cathode electrode and the opposed electrode. This causes the plasma to decompose and excite the reaction gas, whereby a deposited film is formed on the film-deposited substrate (706).
The RF energy of 13.56 MHz is normally used as RF energy. The plasma CVD process with the discharge frequency of 13.56 MHz has such advantages that control of discharge conditions is relatively easy and that quality of the film obtained is excellent. However, it has problems of low utilization factor of gas and relatively low rate of formation of deposited film.
Taking such problems into consideration, investigation has been made on the plasma CVD process using high frequency (RF) waves in the frequency range of approximately 25 to 150 MHz. For example, Plasma Chemistry and Plasma Processing, Vol. 7, No. 3 (1987) p 267-273 describes the process for forming an amorphous silicon (also referred to as "a-Si") film by decomposing raw-material gas (silane gas) by RF energy of frequency of 25 to 150 MHz, using a parallel plate type glow discharge decomposing apparatus. Specifically, it describes that the amorphous silicon films are formed with changing the frequency in the range of 25 to 150 MHz, that the film deposition rate is highest, 2.1 nm/sec, at 70 MHz, which forming rate is approximately 5 to 8 times greater than in the case of the above plasma CVD process using 13.56 MHz, and that defect density, band gap, and electrical conductivity of the amorphous silicon films obtained are rarely affected by excitation frequency. However, film formation described in this reference was carried out in laboratory scale, and it describes nothing about whether such effects can be expected in formation of films in a large area. Further, this reference suggests nothing about efficient formation of large-area semiconductor device for practical use by carrying out film formation simultaneously on a plurality of substrates. This reference simply suggests the possibility that use of RF (13.56 to 200 MHz) will open interesting vistas of high-speed processing of low-cost and large-area a-Si:H thin film devices required to have the thickness of several .mu.m.
The above conventional example is an example of the plasma CVD apparatus suitable for processing the flat plate substrate, while EP-A-154160 describes an example of the plasma CVD apparatus suitable for forming a deposited film on a plurality of cylindrical substrates. This reference discloses the plasma CVD apparatus using the microwave energy source of frequency 2.45 GHz and the plasma CVD apparatus using a radio frequency energy (RF energy) source. In the plasma CVD apparatus using the microwave energy source, the plasma density is extremely high upon film formation because of the use of microwave energy, and therefore, the raw-material gas is decomposed quickly to achieve high-speed deposition of film. This results in a problem that it is difficult in the case of this apparatus to stably form a finer deposited film.
An example of the plasma CVD apparatus (RF plasma CVD apparatus) using the RF energy source, of the type described in EP-A-154160, will be described with reference to FIG. 2 and FIG. 3. FIG. 2 and FIG. 3 illustrate a plasma CVD apparatus based on the RF plasma CVD apparatus described in Reference 2. FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 2. In FIG. 2 and FIG. 3, reference numeral 100 designates a reaction vessel. In the reaction vessel (100), there are six substrate holders (105A) disposed concentrically at predetermined intervals. Numeral 106 denotes cylindrical substrates for film formation disposed on the respective substrate holders (105A). A heater (140) is provided inside each substrate holder (105A) and is arranged to heat the cylindrical substrate (106) from inside. Each substrate holder (105A) is arranged to rotate as being connected to a shaft (131) for rotation of the substrate connected to a motor (132). Numeral 105B denotes auxiliary holding members of cylindrical substrate (106). Numeral 133 represents seal members. Numeral 103 designates the cathode electrode for input of RF power located at the center of plasma generation region. This cathode electrode (103) is connected through a matching circuit (109) to an RF generator (111). Numeral 120 indicates a cathode electrode supporting member. Numeral 107 stands for an exhaust pipe provided with an exhaust valve, and this exhaust pipe (107) is in communication with an evacuation mechanism (135) provided with a vacuum pump. Numeral 108 designates a raw-material gas supply source constructed of a gas bomb, a mass flow controller, a valve, and so on. This raw-material gas supply source (108) is connected through gas supply pipe (117) to gas discharge pipes (116) provided with a plurality of gas discharge holes.
The plasma CVD is carried out as follows using the above apparatus. The reaction vessel (100) is evacuated to a high vacuum by the evacuation mechanism (135) and thereafter the raw-material gas is introduced from the raw-material gas supply source (108) through the gas supply pipe (117) and gas discharge pipes (116) into the reaction vessel (100) to maintain the inside of the vessel under predetermined pressure. After being thus set, the RF power is supplied from the RF power supply (111) through the matching circuit (109) to the cathode electrode (103) to generate the plasma between the cathode electrode and the cylindrical substrates (106). This causes the plasma to decompose and excite the raw-material gas, whereby a deposited film is formed on the cylindrical substrates (106).
Use of the plasma CVD apparatus illustrated in FIG. 2 and FIG. 3 enjoys an advantage that the raw material gas can be used at a high utilization factor, because the discharge space is surrounded by the cylindrical substrates (106).
It is, however, necessary to rotate the cylindrical substrates in order to form the deposited film over the entire surface of cylindrical substrate and the rotation will decrease the substantial deposition rate to about one third to one fifth of that in the case of use of the parallel plate type plasma CVD apparatus described above. The reason is as follows. Since the discharge space is surrounded by the cylindrical substrates, the deposited film is formed at a deposition rate equivalent to that of the parallel plate type plasma CVD apparatus at the position where each cylindrical substrate is opposed to the cathode electrode, whereas little deposited film is formed at positions where the substrate is not in contact with the discharge space. EP-A-154160 describes nothing about specific frequencies of the RF energy. When the amorphous silicon film was actually deposited over the entire circumferential surface of substrate with rotating the cylindrical substrates, using the plasma CVD apparatus shown in FIG. 2 and FIG. 3, using ordinary 13.56 MHz as the RF energy, and using SiH.sub.4 as the raw-material gas, under the pressure condition of several 100 mTorr it was easy to achieve a high deposition rate but to generate powder of polysilane or the like, the substantial deposition rate was at most 0.5 nm/s. In fabricating an electrophotographic photosensitive member having the photosensitive layer of the amorphous silicon film using the plasma CVD apparatus shown in FIG. 2 and FIG. 3, supposing the necessary film thickness of the amorphous silicon photosensitive layer is approximately 30 .mu.m, 16 or more hours will be necessary for deposition of film if the above deposition rate of about 0.5 nm/sec is applied. This is not always satisfactory in productivity. Also, the plasma CVD apparatus shown in FIG. 2 and FIG. 3 tends to form a nonuniform plasma in the axial direction of cylindrical substrate when the frequency of RF energy is 30 or more MHz, and thus has a problem that it is extremely difficult to form a homogeneous deposited film on the cylindrical substrate.